Stephenson Valve Gear - Historical Background

Historical Background

During the 1830s the most popular valve drive for locomotives was known as gab motion in the U.K. and V-hook motion in the U.S.A. The gab motion incorporated two sets of eccentrics and rods for each cylinder; one eccentric was set to give forward and the other backwards motion to the engine and one or the other could accordingly engage with a pin driving the distribution valve by means of the gabs: - vee-shaped ends to the eccentric rods supposed to catch the rocker driving the valve rod whatever its position. It was a clumsy mechanism, difficult to operate, and only gave fixed valve events.

In 1841 two employees in Stephenson’s locomotive works, draughtsman William Howe and pattern-maker William Williams, suggested the simple expedient of replacing the gabs with a vertical slotted link, pivoted at both ends to the tips of the eccentric rods. To change direction, the link and rod ends were bodily raised or lowered by means of a counterbalanced bell crank worked by a reach rod that connected it to the reversing lever. This not only simplified reversing but it was realized that the gear could be raised or lowered in small increments, and thus the combined motion from the “forward” and “back” eccentrics in differing proportions would impart shorter travel to the valve, cutting off admission steam earlier in the stroke and using a smaller amount steam expansively in the cylinder, using its own energy rather than continuing to draw from the boiler. It became the practice to start the engine or climb gradients at long cutoff, usually about 70-80% maximum of the power stroke and to shorten the cutoff as momentum was gained to benefit from the economy of expansive working and the effect of increased lead and higher compression at the end of each strike. This process was popularly known as "linking up" or “notching up”, the latter because the reversing lever could be held in precise positions by means of a catch on the lever engaging notches in a quadrant; the term stuck even after the introduction of the screw reverser. A further intrinsic advantage of the Stephenson gear not found in most other types was variable lead. Depending on how the gear was laid out, it was possible to considerably reduce compression and back pressure at the end of each piston stroke when working at low speed in full gear; once again as momentum was gained and cutoff shortened, so lead was automatically advanced and compression increased, cushioning the piston at the end of each stroke and heating the remaining trapped steam in order to avoid temperature drop in the fresh charge of incoming admission steam.

American locomotives universally employed inside Stephenson valve gear placed between the frames until around 1900 when it quickly gave way to outside Walschaerts motion. In Europe, Stephenson gear could be placed either outside the driving wheels and driven by either eccentrics or return cranks or else between the frames driven from the axle through eccentrics, as was mostly the case in Great Britain.